1. Field of the Invention
The present invention relates to the field of chopper stabilized amplifiers.
2. Prior Art
Frequency compensation for precision OpAmps (operational amplifiers) has a large history. FIG. 1 illustrates the nested Miller compensation scheme of U.S. Pat. No. 4,559,502 for compensation of a 3-stage amplifier (multipath hybrid nested Miller compensation is shown in U.S. Pat. No. 5,486,790). The single output stage does not need frequency compensation. The compensation capacitors CM11 and CM12 compensate for the extra gain of amplifier G2, while CM21 and CM22 compensate for the extra gain of amplifier G3.
For obtaining a low offset, choppers can be placed around the input amplifier G3. This is shown in FIG. 2. The offset of amplifier G3 will appear as a residual square wave at the output as fed back to the input of the amplifier. The average offset is nearly zero.
The chopper stage amplifier G3 requires compensation capacitors CM21 and CM22 similar to the version without choppers of FIG. 1. The chopper amplifier of FIG. 2 has a frequency response corresponding to the lower amplitude characteristic of FIG. 13, but does not have chopper stabilization. The frequency response has a horizontal part with high gain at low frequencies, and a straight 6 dB/oct roll off at higher frequencies. This straight roll off is desired for providing stability with a large choice of gain settings by the feedback network. The slope of 6 dB/oct normally results in a phase margin of 90 degrees.
The situation changes with a chopper-stabilized OpAmp as in FIG. 3. Chopper stabilization is used to compensate for the offset of the OpAmp. A chopper Ch2 is used to sense the offset of the OpAmp and convert this into a square wave. This square-wave signal is converted into a current by voltage to current converter G7 and redirected into a DC current by chopper Ch4. Next, the redirected current is fed into an integrator G6 and then converted again in a current by voltage to current converter G5 and subtracted from the output current of the offset producing input stage G3, so that its offset gets compensated. The choppers and integrator are needed to reduce the influence of the offset of the sense amplifier G7. This offset merely causes a small triangular voltage at the output of integrator G6. A reduced triangular voltage residue is present at the input of the amplifier. The larger the ratio τ=CM61, 62/GM7, where GM7 is the transconductance of the voltage to current converter G7, the smaller the triangular voltage.
The frequency characteristic of the chopper stabilized amplifier of FIG. 3 differs from that of FIG. 2 in that a bubble arises in the amplitude characteristic at low frequencies, as depicted in the upper curve of FIG. 13. The bubble is the result of the extra low-frequency gain caused by the multipath integrator G6. From a certain frequency f3 back to the lower frequency f1, the multipath integrator G6 has more gain than the main path, and its extra pole shows as a 12 dB/oct slope. From frequency f1, where the gain of the main amplifier reaches its flat maximum, there is a 6 dB/oct roll off Back to frequency f2. Finally, when at very low frequencies the leakage of the integration ends the integration function, the frequency characteristic becomes horizontal. If the amplifier is used in high-gain feedback settings, the 12 dB/oct part may cause undesired near-unstable behavior. Therefore, ways to straighten out this bubble are needed.
If the chopper chopper-stabilized architecture of FIG. 4 is used, (U.S. Pat. No. 6,734,723) there is the same frequency bubble as the circuit of FIG. 3. In FIG. 4, the chopper Ch2 has two functions: firstly as input chopper for the main amplifier G3 with output chopper Ch1, and secondly, chopper Ch2 converts the input offset of transconductance amplifier G3 into a square wave referred to the input. Now the offset of amplifier G3 can be sensed by transconductance amplifier G7, independent of the offset of amplifier G7, redirected by chopper Ch4, and integrated by integrator G6 and corrected for by G5. This results in a similar bubble of the frequency characteristic as that of FIG. 4,: which is shown in FIG. 13. Therefore, there is a need to straighten the bubble out in this case also.
FIG. 5 illustrates a prior art multipath operational amplifier structure having four stages in the gain path. This gain path is hybrid nested compensated. This means that it has a backward nest CM11 and CM12 around amplifier G1 at the output, and a forward nest CM61 and CM62 around amplifier G6, both encompassed by an outward nest CM51 and CM52 around amplifier G5. It also has a multipath amplifier G3 in order to increase the bandwidth. This results in straight frequency roll off with 6 dB/oct if the dimensioning is done well.